A new optical computer input system is shown and described in said parent patent application. Such system enable the use to shine a high intensity light onto a screen bearing a computer generated image to provide auxiliary information for the computer. Such an input system includes an optical sensing device, such as a charged coupled device camera focused on to the screen. Thus the system can detect high intensity light images and discriminate them from the computer generated images, to input information interactively into the computer, in a convenient manner, even in very low ambient light conditions.
While such a computer input system and method of using it has proven to be highly satisfactory, it would be desirable to provide for a geometric compensation or correction. There are various reasons why such a geometric correction is required. First, the screen onto which is projected the computer generated image may not be a perfect rectangle as presented to the sensing device or camera. In this regard, the screen may be tilted either forward or backward, or from side to side, or any combination thereof. Thus, the sensing device or camera will not track properly relative to the image visualized from the screen.
An even more significant problem is the problem of "keystoning" which is caused by an overhead projector utilized in projecting the computer generated image onto the screen. In this regard, the commonly known keystoning problem produces an image which has a longer top edge as compared to its bottom edge. Such keystoning problem is well known with overhead projectors, and thus, the sensing device or camera will produce a distortion in sensing the image projected into the computer.
A third problem is caused by the improper alignment of a projection panel on the stage of the overhead projector. In this regard, if the panel is not accurately aligned in a parallel manner on all sides relative to the projector's stage, the resulting image projected onto the screen will also be askew.
A fourth problem relates to the project itself not being properly aligned relative to the screen. Such is commonly the case where the neck portion of the overhead projector may be bent slightly due to excessive use or wear. This causes a result similar to the improper registration of the panel on the stage of the projector.
A still further problem of geometric alignment is caused by the camera or sensing device being tilted at an angle being tilted upwardly, or downwardly, relative to the plane of the screen. The result is that a distortion may occur.
As a result of both described distortions due to the geometry of the screen, projector and panel, as well as the camera itself, the camera is unable to accurately plot the various coordinates visualized from the image projected on to the screen. As a result, tracking is not able to be perfectly accomplished. Thus, when a light is projected onto the screen, the camera may not accurately know the precise coordinates of the spot of light projected onto the screen. As a result, the computer may not accurately respond to the position of the light and incorrect data can be entered. Thus, erroneous results might occur.